The Story of the Rocks 137 



mens, which are therefore on exactly the same scale. 

 The single dinosaurian tooth greatly exceeds net only 

 the tooth of the mammal, but the containing jaw or 

 even the entire creature as the imagination conjurer it 

 up." 2 



As to the cause of mammalian development we can again 

 only conjecture. Lull has suggested that increasing dry ness 

 of climate and corresponding desert conditions, necessitat- 

 ing speed on the part of animals in search of food and water, 

 or in night from their enemies, coupled with the extensive 

 glaciation in the Southern hemisphere in late Palaeozoic t:mes, 



TOOTH OF A DINOSAUR COMPARED WITH THE JAW OF A CONTEMPORARY 



MAMMAL 



From Lull's "Organic Evolution." 

 Courtesy of Professor Lull and the Macmillan Company. 



which glaciation would mean increasing cold and the nred 

 of a furry covering, were the inciting causes. But apart 

 from the fact that this explanation implies, if it does not 

 state, an acceptance of Lamarck's doctrine, which at the 

 present time is in the discard with most zoologists, is the 

 further fact that the succeeding or Mesozoic era was one 

 which witnessed the remarkable development of reptiles, 

 which are distinctly types not adapted to aridity and cold- 

 ness of climate. Perhaps the best we can do after all, when, 

 as so frequently happens in philosophy and science, we find 



3 Lull, "Evolution of the Earth," pp. 133-134. By permission of the 

 Yale University Press. 



